1. Field of the Invention
The present invention relates to an apparatus for automatically compensating for the focus and the amount of displacement using an electron microscope image.
2. Description of the Prior Art
The inventor of the present invention have searched the Prior Art with respect to apparatuses for compensating for the focus and the amount of displacement by determining whether or not to automatically compensate therefor by using an electron microscope image, or methods for compensating for the amount of displacement of a continuously moving specimen stage. As the result of search, there have been found three relevant topics. First, a paper entitled as “The correction of image drift for autotuning a TEM using phase spectrum” by Norihiko Ichise et al., disclosed in the proceedings of the 51st academic conference of Japan Electron Microscope Association, May 1995, pp. 161, discloses a method for analyzing and compensating for the influence of image drift by using the phase spectrum method for analyzing the focus, astigmatism, and shifted axis. However, this paper does not disclose anything about the improvement of the precision of analysis by means of the computation of the gravity center of a peak, and determination by means of compensation values, as well as about the compensation of the focus and drift of a continuously moving specimen stage. Second, the Japanese Unexamined Patent Publication No. Hei 10-187993, discloses an apparatus for analysis of displacement between images by using the phase variance of Fourier transform images on two photos taken in different conditions.
However, this application does not disclose anything about the feedback to an electron microscopy apparatus and the spirit of the art but only the measurement of shape and distance of an object from a mark attached to the object. Third, the Japanese Unexamined Patent Publication No. Hei 10-339607, discloses the detection of amount of displacement by image processing of the parallax of electron microscopy images, and the feedback of the result thereof to the electron microscopy apparatus. More specifically, the images do not move before and after the angle of incidence of electron beam varies if a specimen is located just on the focus plane, however, if the specimen is located out of focusing plane then the images move before and after the angle of incidence of electron beam. The relationship between the displacement D and out-of-focus F is D=M α (F+Cs α2), where a designates to the deflection angle of incident electron beam, M to the magnification rate, Cs to the coefficient of spherical aberration, therefore the out-of-focus F may be given if the parallax displacement D is determined. There is disclosed an apparatus for compensating for the focusing of objective lens system by storing on a memory a pair of images before and after altering the incident angle to apply a cross-correlation method or least-squares estimation method to analyze the displacement D to determine the amount of defocus F. However, the analysis method of displacement using the phase variance of Fourier transform images is not disclosed. In an apparatus for automatically compensating for the focusing or the amount of displacement by using electron microscopy images, the performance may depend on the settings of the photographic condition of images, analyzing images, and feeding back the analysis results. However, the optimization is not performed with respect to the object, precision and time of compensation.
The performance of the apparatus for automatic focusing an electron microscope in accordance with the displacement D between electron microscopy images such as the focus analysis using parallax and the like may depend largely on the analysis method of displacement D. The analysis methods of displacement used heretofore, such as for example the cross-correlation method, least-squares method and the like, were limited in terms of the precision by size of pixels of the electron beam detector. The length of a side of pixel of a CCD camera used for present electron microscopy imaging is approximately 25 microns. The amount of defocus F corresponding to a pixel may depend on the angle and magnitude of incident electron beam, the variance of incident angle α may be approximately at most 0.5° due to the limitation by the hole diameter of objective aperture, and the magnitude should be the actual observation magnitude. For example, at a magnification of 5,000, and an incident angle variance of 0.5°, the focusing distance corresponding to the displacement D of a pixel is approximately 0.6 microns. This value is less than the precision level of focus compensation by a skilled operator. The improvement of performance of apparatus such as refining the images used by the displacement analysis in favor of improving the precision in the focusing analysis may cause excessive time spent for analysis and excessive cost of hardware, thus is not practical.
The conventional method of displacement analysis has no functionality of numerical verification on whether or not an analysis has been performed correctly, so that the operator had to guess the result by the eye. Otherwise the operator had to compensate for the focusing based on thus obtained analysis results to verify that the compensation was done accurately. Since the automatic compensator has not assurance enough to correctly perform any analysis, there may be a need for a functionality of aborting compensation when the result of analysis is not highly reliable.
Furthermore, in the analysis of displacement in the prior art the analysis was almost impossible when the image was shadowed by the objective aperture. This phenomenon may be encountered routinely in the TEM observation, malfunction thereby thus may cause problems in practice.